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Possible SHX8 CV Multiplexer solution - Transconductance Amplifiers..

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Hi,

Heres something which may give some pointers for a way to make a replacement CV multiplexer.

Transconductance OpAmps can be used for a great many things (almost every analog synth circuit you could think of is in the following pdf) ..one of which is Sample & Hold.

Please see National semi lm13700 - Page 18, fig. 20 for details as to how this can be done.

http://cache.national.com/ds/LM/LM13700.pdf

It might actually be practical to use such opamps; provided well-packed chips can be found :) ..not as neat a solution as the NJD's ..but oh well !

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Hi,

Using OTAs isn´t an elegant or Cost Effective solution. It´s too bad those NJDs are obsolete. (other available S&H chips I´ve found really aren´t up to the job)

I still think using 4051 multiplexers with a hold cap and buffering OpAmps would be easiest way to go. It´s a cheap solution, but uses pretty much board space.

Moebius

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true ...i was hoping for some transconductance opamps with more than 2 in a package ..couldnt find any tho.

Was however, on a unrelated matter looking at multiplexer switchers.. and well, you beat me to that conclusion :)

yes, cap thing should work i guess, provided that the charge leakage is very low ..otherwise static signals stored might drift. However i beleive it is possible to prevent this happening with the correct configuaration of negative feedback to stabalise against any really slow drifts.

(the reason i was proposing Transconductance opamps, is that it would eliminate the need for a multiplexing switcher.. as you can already do that with just this opamp... tho just cant seem to get enough of them stuffed into one chip)

as for compactness.. hell its worth doing it surface mount if it came to it... could even use a double sided board & put chips on both sides as independant circuits. ...then just drill holes & drop components like resistors in ..its a tad more skilled effort ...tho damn id do it, if it would fit 64 channels into a board the size of a fag packet & 1/2-1/3 the height :)

On the other reason for looking at Multiplexing switchers, the next little project im working on is a Midi controlled (CV) FilterBank. ..and i want to be able to route the signal paths inside it. In this case im intent on using a soundcard with 8 to 16 In+outs ...and using the swithers to route the signals through the filters i want to use... then returning them to the computer for further mixing & recording (saves on blowing loads of money on audio channels which arent being used most of the time).

Makes for a nice way to insert real analog magic into the 'infinate' synthsising Potential of a digital enviroment :)

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I still think using 4051 multiplexers with a hold cap and buffering OpAmps would be easiest way to go. It´s a cheap solution, but uses pretty much board space.

Yeh but if it's mounted in a case big enough for 64 1/4" jacks I'm not sure it'll matter all that much ;)

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true... tho most of the CV connections in my case will be internal ribbon cables going to very tightly stacked arrays of filter boards, etc :)

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:D Correction: Yeh but if it's mounted in a case big enough for 64 filter boards I'm not sure it'll matter all that much ;) hehehe

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aahhem... that would be assuming that you only intended to control the filters cutoff ..what about other thing like resonance ? :) ...doing that brings the number to 32 boards. ..which do indeed fit comfortably into 2u ...with a little insanitising.

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Hi,

yes, cap thing should work i guess, provided that the charge leakage is very low ..otherwise static signals stored might drift. However i beleive it is possible to prevent this happening with the correct configuaration of negative feedback to stabalise against any really slow drifts.

There are no "static signals" - S&H is constantly updated with current voltage in the DAC output. NJU7304 was pretty good at droop rate of 1mV/5ms.

Of course "low leakege" is needed, but I can´t see reason to use OTA (with hold cap and buffer OpAmp) as analog switches already have MegaOhm off state, so it´s more up to the cap and switching rate to hold the voltage.

Moebius

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ahh ...serves me right for thinking about these things at 3am ...yeah, given that the outputs are cyclically updated.. arresting drift is not nessursary.

Hmm it occurs to me ..what we're dealing with here is an analog data bus addressing analog memory ...how about that :)

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Hi,

Hmm it occurs to me ..what we're dealing with here is an analog data bus addressing analog memory ...how about that :)

"With digital control, to address the drift" - It´s a nice definition!

Moebius

p.s. TK, please save me - what is the update frequency of the current S&H code?

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Wish I had caught this one a few days ago...  I don't think there are any OTAs out there with more than two per package.  The most common ones are/were CA3080, CA3280 (one per), and the LM13600/13700 (two per), and various clones.

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Since i'm not able to find the nju7403 chip i'm also looking for an alternative

chip to build a SH8X module.

The 4051 multiplexer route should work fine, but it is a lot of parts to build;

8 multiplexers plus an opamp and cap for each of the 64 cv's.

I stumbled upon a comparable chip from Analog Devices, the SMP08 (or SMP018):

http://www.analog.com/en/prod/0,,773_925_SMP08%2C00.html

The truth table is the same and it's an all in one chip like the nju.

It is not cheap, but not much more expensive than the nju.

Anyone ever tried this or can give me a clue why this would not work

as a direct replacement (except for the pin layout)?

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hmm.. damn good spot, best alternative so far after a cursory look :)

anyone like to offer a pro hw geek opinion on the analog devices multiplexer there ? :)

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Woops I bumped it. ;)

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